Mechanisms for enzymatic reduction of nitric oxide to nitrous oxide - A comparison between nitric oxide reductase and cytochrome c oxidase.

Abstract Cytochrome c oxidases (C c O) reduce O 2 to H 2 O in the respiratory chain of mitochondria and many aerobic bacteria. In addition, some species of C c O can also reduce NO to N 2 O and water while others cannot. Here, the mechanism for NO-reduction in C c O is investigated using quantum mechanical calculations. Comparison is made to the corresponding reaction in a "true" cytochrome c -dependent NO reductase (c NOR). The calculations show that in c NOR, where the reduction potentials are low, the toxic NO molecules are rapidly reduced, while the higher reduction potentials in C c O lead to a slower or even impossible reaction, consistent with experimental observations. In both enzymes the reaction is initiated by addition of two NO molecules to the reduced active site, forming a hyponitrite intermediate. In c NOR, N 2 O can then be formed using only the active-site electrons. In contrast, in C c O, one proton-coupled reduction step most likely has to occur before N 2 O can be formed, and furthermore, proton transfer is most likely rate-limiting. This can explain why different C c O species with the same heme a 3 -Cu active site differ with respect to NO reduction efficiency, since they have a varying number and/or properties of proton channels. Finally, the calculations also indicate that a conserved active site valine plays a role in reducing the rate of NO reduction in C c O. Graphical Abstract Highlights • Mechanisms for NO reduction in C c Os with a heme a 3 active site are studied. • Free energy profiles are constructed combining computational and experimental data. • Comparisons are made to the mechanism for NO reduction in cNOR. • High reduction potentials in C c Os are found to cause low rates of NO disappearance. • A conserved valine may play a role in reducing the NO reduction rate in C c O. [ABSTRACT FROM AUTHOR]